160 research outputs found
Viscous relaxation and collective oscillations in a trapped Fermi gas near the unitarity limit
The viscous relaxation time of a trapped two-component gas of fermions in its
normal phase is calculated as a function of temperature and scattering length,
with the collision probability being determined by an energy-dependent s-wave
cross section. The result is used for calculating the temperature dependence of
the frequency and damping of collective modes studied in recent experiments,
starting from the kinetic equation for the fermion distribution function with
mean-field effects included in the streaming terms.Comment: 10 pages, 9 figures; proof version, corrected typo in Eq. (23);
accepted for publication in PR
Synthetic gauge fields in synthetic dimensions
We describe a simple technique for generating a cold-atom lattice pierced by
a uniform magnetic field. Our method is to extend a one-dimensional optical
lattice into the "dimension" provided by the internal atomic degrees of
freedom, yielding a synthetic 2D lattice. Suitable laser-coupling between these
internal states leads to a uniform magnetic flux within the 2D lattice. We show
that this setup reproduces the main features of magnetic lattice systems, such
as the fractal Hofstadter butterfly spectrum and the chiral edge states of the
associated Chern insulating phases.Comment: 5+4 pages, 5+3 figures, two-column revtex; v2: discussion of role of
interactions added, Fig. 1 reshaped, minor changes, references adde
Collective excitations of a trapped Bose-Einstein condensate in the presence of a 1D optical lattice
We study low-lying collective modes of a horizontally elongated 87Rb
condensate produced in a 3D magnetic harmonic trap with the addition of a 1D
periodic potential which is provided by a laser standing-wave along the
horizontal axis. While the transverse breathing mode results unperturbed,
quadrupole and dipole oscillations along the optical lattice are strongly
modified. Precise measurements of the collective mode frequencies at different
height of the optical barriers provide a stringent test of the theoretical
model recently introduced [M.Kraemer et al. Phys. Rev. Lett. 88 180404 (2002)].Comment: 4 pages, 4 figure
Strong-coupling ansatz for the one-dimensional Fermi gas in a harmonic potential
A major challenge in modern physics is to accurately describe strongly interacting quantum many-body systems. One-dimensional systems provide fundamental insights since they are often amenable to exact methods. However, no exact solution is known for the experimentally relevant case of external confinement. Here, we propose a powerful ansatz for the one-dimensional Fermi gas in a harmonic potential near the limit of infinite short-range repulsion. For the case of a single impurity in a Fermi sea, we show that our ansatz is indistinguishable from numerically exact results in both the few- and many-body limits. We furthermore derive an effective Heisenberg spin-chain model corresponding to our ansatz, valid for any spin-mixture, within which we obtain the impurity eigenstates analytically. In particular, the classical Pascal's triangle emerges in the expression for the ground-state wavefunction. As well as providing an important benchmark for strongly correlated physics, our results are relevant for emerging quantum technologies, where a precise knowledge of one-dimensional quantum states is paramount
Twin peaks in rf spectra of Fermi gases at unitarity
We calculate the radio-frequency spectrum of balanced and imbalanced
ultracold Fermi gases in the normal phase at unitarity.
For the homogeneous case the spectrum of both the majority and minority
components always has a single peak even in the pseudogap regime.
We furthermore show how the double-peak structures observed in recent
experiments arise due to the inhomogeneity of the trapped gas.
The main experimental features observed above the critical temperature in the
recent experiment of Schunck et al. [Science 316, 867, (2007)] are recovered
with no fitting parameters.Comment: v3: version accepted for publication as a Rapid Communication in PRA.
With respect to v2, minor changes in the text and in the inset of Fig.
Spin polarons and molecules in strongly-interacting atomic Fermi gases
We examine pairing and molecule formation in strongly-interacting Fermi
gases, and we discuss how radio-frequency (RF) spectroscopy can reveal these
features.
For the balanced case, the emergence of stable molecules in the BEC regime
results in a two-peak structure in the RF spectrum with clearly visible medium
effects on the low-energy part of the molecular wavefunction.
For the highly-imbalanced case, we show the existence of a well-defined
quasiparticle (a spin polaron) on both sides of the Feshbach resonance, we
evaluate its lifetime, and we illustrate how its energy may be measured by RF
spectroscopy.Comment: 4 pages, 5 figures. Revised version accepted for publication: minor
changes to Fig. 2 (added inset with the chemical potential at unitarity), to
Fig. 3 (experimental data updated), and to the notatio
Decay of polarons and molecules in a strongly polarized Fermi gas
The ground state of an impurity immersed in a Fermi sea changes from a
polaron to a molecule as the interaction strength is increased.
We show here that the coupling between these two states is strongly
suppressed due to a combination of phase space effects and Fermi statistics,
and that it vanishes much faster than the energy difference between the two
states, thereby confirming the first order nature of the polaron-molecule
transition. In the regime where each state is metastable, we find quasiparticle
lifetimes which are much longer than what is expected for a usual Fermi liquid.
Our analysis indicates that the decay rates are sufficiently slow to be
experimentally observable.Comment: Version accepted in PRL. Added discussion of three-body losses to
deeply bound molecular state
A strongly interacting Bose gas: Nozi\`eres and Schmitt-Rink theory and beyond
We calculate the critical temperature for Bose-Einstein condensation in a gas
of bosonic atoms across a Feshbach resonance, and show how medium effects at
negative scattering lengths give rise to pairs reminiscent of the ones
responsible for fermionic superfluidity. We find that the formation of pairs
leads to a large suppression of the critical temperature. Within the formalism
developed by Nozieres and Schmitt-Rink the gas appears mechanically stable
throughout the entire crossover region, but when interactions between pairs are
taken into account we show that the gas becomes unstable close to the critical
temperature. We discuss prospects of observing these effects in a gas of
ultracold Cs133 atoms where recent measurements indicate that the gas may be
sufficiently long-lived to explore the many-body physics around a Feshbach
resonance.Comment: 8 pages, 8 figures, RevTeX. Significantly expanded to include effects
beyond NS
Energy-dependent effective interactions for dilute many-body systems
We address the issue of determining an effective two-body interaction for
mean-field calculations of energies of many-body systems. We show that the
effective interaction is proportional to the phase shift, and demonstrate this
result in the quasiclassical approximation when there is a trapping potential
in addition to the short-range interaction between a pair of particles. We
calculate numerically energy levels for the case of an interaction with a
short-range square-well and a harmonic trapping potential and show that the
numerical results agree well with the analytical expression. We derive a
generalized Gross--Pitaevskii equation which includes effective range
corrections and discuss the form of the electron--atom effective interaction to
be used in calculations of Rydberg atoms and molecules.Comment: 6 pages, 2 figure
- …